1. Field of the Invention
The present invention relates to a photomask used by photolithography, and a process for making same.
2. Description of the Prior Art
In recent years, semiconductor technology has made remarkable progress. In line with this progress, highly intricate patterns have been required for sophisticated semiconductor devices, such as LSIs. The formation of intricate or highly fine patterns is entirely due to the recent development of lithography.
Lithography consists of a process in which a resist is first coated, then exposed to light and developed with a developer. The fineness of the resulting patterns has been improved by the development of the material of the resist, the device and method of exposure.
There are proposals for reducing the size of patterns, which, for example, are disclosed in Japanese Patent Publication No. 62-50811 and No. 62-59296. The feature common to these proposals is that a photomask used for exposing radiation is improved so that a phase difference arises in the irradiated light or incident light upon the photomask through an optical system, thereby achieving fine, intricate patterns.
Referring to FIGS. 26 and 27, the known photomask will be described in greater detail:
A photomask substrate 1 is prepared, and a light-shielding layer 2 is formed thereon so as to give a circuit pattern to be transferred. Transparent or transmissive layers 3 are provided on either side of the light-shielding layer 2. One of them acts as a phase shifter. The phase shifter is to change the phase of an exposing radiation.
In FIG. 27, the substrate 1 is composed of two different members 4 and 5 arranged side by side, of which the members 4 are transparent or transmissive layer. The members 4 act as phase shifters. This example is characteristic in that the phase shifter 4 is not formed on the substrate 1. This type of photomask is disclosed in Japanese Laid-Open Patent Publication No. 62-92438.
These prior art photomasks have the following disadvantages:
In the photomask shown in FIG. 26 the light-shielding pattern is formed in a conventional photomask making process; more specifically, a substrate is wholly covered with a thin layer so as to form a light-shielding layer pattern. This thin layer is made of chromium layer or molybdic silicide layer. The thin layer is then coated with a resist, and baked. Then a desired area is exposed to light or electron beams. The resist in the exposed area is removed or remain by using an appropriate developer so as to form a resist pattern. Finally, a liquid or gaseous etchant is used, called wet etching or dry etching, to etch the thin layer into a light-shielding pattern.
Then, a phase shifter 4 of a thin film is formed on the light-shielding pattern and a transparent area outside the light-shielding area. The thin layer is made of photosensitive resin. Light or electron beams are radiated on a desired area in the thin layer. It is necessary to align the exposing radiation with the edges of the light-shielding patterns. This means two times of exposure, thereby resulting in a complicated exposing procedure. In addition, the alignment must be strictly accurate, otherwise a desired phase shifter will not be attained.
In general, a photomask must be perfect, free from any defects. However, it is difficult to detect any fault in the photomask by scanning light so as to find a scattering light or a permeating light. This detecting method is likely to be inaccurate because of an out-of-focus irradiation caused by the uneven thickness of the phase shifter due to the presence of the light-shielding layer, and because of the impermeability of light in the overlapping of the light-shielding layer over the phase shifter. In this case it is necessary to produce another phase shifter so as to remedy the defects.
In the phase shifter shown in FIG. 27 the phase shifters are arranged at equal intervals. In order to achieve a desired phase difference, it is necessary to fabricate the intervals to a high precision, thereby resulting in a difficult fabrication of photomasks. Irregular intervals cannot produce fine, intricate patterns suitable for IDs used in LSIs.